1. Field of the Invention
The present invention relates generally to a method and a device for acoustic communication in which digital data is transmitted among mobile devices using acoustic signals, and in particular, to a method and a device for acoustic communication using a psychoacoustic model.
2. Description of the Related Art
Acoustic communication is one of the possible ways to transfer digital information between mobile devices. An advantage of acoustic communication is that the data communication protocols can be implemented on existing devices using only software without having to add any hardware elements such as antenna and RF front-end, as required for radio-based communication systems.
Several methods have been proposed to mask acoustic communication by music or speech signals to make the acoustic communication sound pleasant to the human ear and to convey additional human-understandable information. Such methods include “echo-hiding” or adding spread-spectrum signal below noise level, as discussed in D. Gruhl, et al., Echo Hiding, Proceedings of the First International Workshop on Information Hiding, Cambridge, U.K., May 30-Jun. 1, 1996, pp. 293-315, and L. Boney, et al., Digital watermarks for audio signals, IEEE Intl. Conf. on Multimedia Computing and Systems, pp. 473-480, March 1996, respectively.
FIG. 1 illustrates a conventional method for mixing an audio program with an acoustic communication signal. A device 100 for implementing such method includes an acoustic communication signal generator 110, a combiner 120 and a speaker 130. In the above method, a low level communication signal such as a spread spectrum signal is simply added to the audio program such as music, speech, alarm sound or the like. The audio program and the acoustic communication signal output from the acoustic communication signal generator 110 are combined (or mixed) by the combiner 120. The combined signal is radiated in a form of sound waves through the speaker 130.
Unfortunately, conventional methods fail to fully exploit the capacity of an acoustic communication channel, and therefore achieve only very low bit rates, i.e. several bits per second.
A better method, such as the type described by Y. Nakashima, et al., in Evaluation and Demonstration of Acoustic OFDM, Proc. Fortieth Asilomar Conference on Signals, Systems and Computers, 2006. ACSSC 2006, pp. 1747-1751, is based on replacement of high frequency components of speech/music audio program with spectrally shaped communication signal.
FIG. 2 is illustrates a method for generating an audio signal mixed with an acoustic communication signal using the known frequency replacement technology. A device 200 for implementing such method includes a Fast Fourier Transform (FFT) block 210, a band splitter 220, an Inverse Fast Fourier Transform (IFFT) block 230, a Forward Error Correction (FEC) coding block 240, an Orthogonal Frequency Division Multiplexing (OFDM) modulator 250, a combiner 260 and a speaker 270.
The FFT block 210 performs FFT on the original audio signal (or program) such as music or speech. Hereinafter, the band splitter 220 divides the FFT audio signal into high frequency bins and low frequency bins, outputs the low frequency bins to the IFFT block 230, and outputs the high frequency bins to the OFDM modulator 250. The IFFT block 230 performs the IFFT on the original audio signal, from which the high frequency bins are removed.
The FEC coding block 240 performs FEC coding on the input digital data and outputs the data. The OFDM modulator 250 performs OFDM on the coded digital data according to the high frequency bins and outputs the data, and the acoustic communication signal from the OFDM modulator has a spectral envelope which is shaped similar to the high frequency bins. In other words, the high frequency bins are replaced with the acoustic communication signal.
FIGS. 3A and 3B illustrate signals which are generated according to the frequency replacement technologies. FIG. 3A shows the frequency spectrum of an original audio signal 330, and FIG. 3B shows the frequency spectrum of a modified audio signal 330a which has a replacement acoustic communication signal. In each frequency spectrum, the frequency is shown along the horizontal axis, and the signal strength is shown along the vertical axis. As shown in FIG. 3A, the original audio signal 330 is divided into the high frequency bins (or region) 320 and the low frequency bins 310 based on frequency division. As shown in FIG. 3B, the low frequency bins 310 of the modified audio signal 330a are the same as those of the original audio signal, and the high frequency bins 320 of the original audio signal are replaced with the acoustic communication signal 325 of the modified audio signal.
This method allows for simple implementation of an acoustic signal receiver since the original audio signal and the acoustic communication signal are transmitted in separate frequency bands. This method, however, has two drawbacks.
Firstly, the method degrades the quality of the original audio signal, i.e. the music/speech signal, because there is a sharp transition in frequency domain between the original audio signal and the acoustic communication signal, see FIG. 3B.
Secondly, this method fails to fully utilize available signal bandwidth, since the acoustic communication signal only concentrates in relatively high audio frequencies. Consequently, if the music/speech audio program does not contain high frequency bins, or if the receiving device microphone is not capable of capturing the entire wideband audio spectrum, including high frequency bins, the acoustic data communication shall be impossible (even with reduced bit rate).